Vortex Generator Flight Tests — Stall Effects
by David F. Rogers
http://www.nar-associates.com
Vortex generators (VGs) are usually associated with
multiengine, STOL or military aircraft. On multiengine aircraft, e.g., a Baron, the principal attraction is a significant improvement in single engine
handling qualites. Recently, a number of STCs have
become available for single engine aircraft including
one from Beryl D’Shannon (BDS) for the Bonanza.
Vortex generators work by re-energizing the flow in
the boundary layer, that thin layer of air of reduced
airspeed right next to the surface. It is separation
of the boundary layer from the upper surface of the
wing that causes stall. Re-energizing the flow in the
boundary layer delays stall and increases the stall
angle of attack. The result is a higher wing lift coefficient and hence a lower stall velocity. Used in
front of or on control surfaces, e.g., ailerons, the
vertical tail/rudders or the horizontal tail, they increase control effectiveness at slower speeds.
The BDS kit consists of 39 VGs installed approximately 10% behind the leading edge of each
wing. Eleven VGs are installed on each wing ahead
of the inboard and outboad ends of the ailerons, and
12 VGs are installed on each side of the vertical tail
ahead of the rudder for a total of 124 VGs in two
different sizes (see Figure 1). The STC installation
Figure 1. Vortex generators.
instructions are very clear and detailed. Detailed
templates are provided to precisely locate the VGs
on both the wing and the vertical tail (shown in
Figure 2). Complete installation was accomplished
in approximately eight hours. The installed VGs are
shown in Figures 3 and 4. An optional 100 lb. gross
weight increase is available. However, the STC for
the gross weight increase changes the aircraft from
utility category (4.4 gs) to normal category (3.8 gs)
when operating at the increased gross weight.
Stall characteristics The straight tail and the
later V-tailed Bonanzas have two stall/spin strips
Figure 2. Vertical tail installation templates. VGs layed out
on the horizontal tail for installation on the vertical tail.
Figure 4. Vortex generators installed on wing.
Figure 3. Vortex generators installed on vertical tail.
mounted on the inboard section of the wing (see Figure 5). The inboard stall/spin strip (black arrow),
mounted at the crank of the wing, activates at a
higher angle of attack than the outboard six inch
long stall/spin strip that looks like a small piece of
angle iron (white arrow).
As the aircraft is pitched up the airflow comes
from underneath the outboard stall/spin strip. If
the airflow comes from underneath the sharp edge
of the stall/spin strip, the local airflow immediately
behind the stall/spin strip separates from the wing
and flows aft in a narrow turbulent stream. Because, as shown in Figure 5, the elevator balance
horn is located immediately behind the small outboard stall/spin strip, the narrow stream of separated turbulent air impinges on the horn and shakes
the elevator and hence the control yoke — in effect
it acts as an aerodynamic stick shaker.
As the aircraft continues to pitch up at a moderate rate, the inboard stall/spin strip located at
the crank of the wing activates. Here a stronger locally separated turbulent flow is generated which,
as Figure 5 indicates, impinges on the center of the
elevator and literally shakes the entire aircraft with
a significant up and down pitching motion. If the
aircraft continues to pitch up at a moderate rate,
a full stall occurs. Hence, with moderate or faster
pitch-up rates there are two significant warnings of
an impending stall. This constitutes good design.
Flight tests without VGs The no VG flight
tests were conducted by a single pilot at pressure
altitudes from 4000 to 8000 feet. Takeoff weight was
2861 lbs. The weight during the flight tests varied
Figure 5. Stall/spin strips.
from 2839 to 2746 lbs. The center of gravity was
80 inches aft of the datum. Tests were conducted
at bank angles from zero to thirty degrees both to
the left and to the right. Power off tests were conducted at idle power in descending glides. Power
on tests were conducted both at full throttle and
2700 RPM and in the power approach configuration (2300 RPM and approximately 23 inches of
manifold pressure or full throttle). All tests were
conducted using a slow approach to stall, i.e., a deceleration of one knot per second or less as specified
in FAR 23.201. Fuel burn was from the left tank.
In the power off clean configuration (gear and
flaps retracted), flight test results show that for
zero bank angle the aircraft stalls at 61+1/−0 miles
per hour indicated airspeed (MIAS). Noting that
the POH shows negligible airspeed correction at 60
mph, at the test weight the POH gives a stall speed
of 66.7 MIAS at 2800 lbs. The most likely explanation for the difference in stall speeds is the FAR
requirement that the value in the POH be for the
worst case (FAR 23.49), which is typically for the
center of gravity at the forward limit of 77 inches.
In both right and left 20◦ banks, stall occurred
at 64 MIAS, while in right and left 30◦ banks stall
occurred at 69±1 MIAS. In all cases a mild wing
drop (typically to the right) occurred or the aircraft
gently bobbled up and down while descending. In
all cases, solid aileron control was maintained up to
and into the stall. This is not surprising, in view of
the fact that both experimental tuft and analytical
studies show that on the Bonanza wing stall begins
at the trailing edge 1–2 feet outboard of the root
and smoothly progresses forward and outward on
the wing. The flow on the wing in the region of the
ailerons does not begin to separate until the stall
is nearly fully developed on the inboard portions of
the wing. Stall recovery was initiated by returning
the control column to a neutral position. Power was
not added.
In the power off dirty configuration (gear and
flaps extended) flight test results show that for zero
bank angle the aircraft stalls at 53+0/−1 MIAS. In
20◦ left and right banks stall occurs at 55+0/−1
MIAS, while in a 30◦ left bank stall occurred at
59 MIAS. At stall a mild left wing drop or gentle
bobble occurred. Aileron control was good.
At full power at 7500 feet pressure altitude (22
inches MP and 2700 RPM) in the clean configuration with zero bank angle, stall occurred at an
average of 55 MIAS while climbing in excess of 500
fpm. Full right rudder was required. In the power
approach configuration (full throttle – 21.5 inches
MP and 2300 RPM) at 7800 feet, gear and flaps
extended with zero bank angle, stall occurred at 54
MIAS. Full right rudder as well as some right aileron
was required to overcome a left turning tendency.
Good control was maintained. Prior to the stall the
aircraft continued to climb. Upon stall the aircraft
rolled to the right. In 20◦ left and right banks the
aircraft also stalled at 54 MIAS. Again, full right
rudder as well as some right aileron was required to
overcome a left turning tendency. Good control was
maintained. Prior to the stall the aircraft continued
to climb. Upon stall a mild left wing drop occurred.
Flight tests with VGs The flight tests with
the VGs installed were conducted under the same
conditions as those without VGs installed. In the
power off clean configuration for zero bank angle
the aircraft stalled at 53 ± 1 MIAS with the VGs
installed, which represents a reduction of 8 MIAS
compared to without VGs. The stall was not as well
behaved as without the VGs installed. The resulting
wing drop was more aggressive. The stall warnings
generated by the stall/spin strips were significantly
decreased. The most likely explanation for the decreased stall warning is that the VGs cause the separated flow from the stall/spin strips to reattach to
the wing until significantly higher angles of attack.
At these higher angles of attack the wing stalls more
abruptly with the VGs installed.
In the power off dirty configuration with VGs installed the aircraft stalled at 50±1 MIAS, which represents a reduction of 3 MIAS compared to without
VGs installed. In this configuration stall occured
with the column full aft. Using 25◦ of trim, i.e., all
available trim, was insufficient to reduce the stick
force to zero. With the ball centered, the aircraft
bobbled upon stalling. This observation, along with
the small difference in stall speed with the VGs installed and the full aft column, suggests that, using
the slow stall technique, a fully developed stall may
not have occurred because of lack of elevator power.
Vortex generators on the underside of the horizontal
stabilizer ahead of the elevators to increase elevator
power may be indicated. Aileron control was solid.
At full power (22 inches MP, 2700 RPM, OAT
of 45◦ F at 7600 feet pressure altitude) in the clean
configuration with zero bank angle, stall occurred
at 52 ± 1 MIAS while the aircraft was climbing.
Full right rudder and partial right aileron was insufficient to prevent a left turning tendency. In the
dirty configuration with zero bank angle stall occurred at 47 ± 1 MIAS. Full right rudder with partial right aileron was necessary. Aileron control was
solid. During stall, significant wing drop of as much
as 45◦ occurred.
In the power approach configuration (23 inches
MP and 2300 RPM) gear and flaps extended with
an OAT of 50◦ F at 7500 feet pressure altitude with
zero bank angle the aircraft stalled at 50 + 1/−2
MIAS while climbing. With the same power settings in the clean configuration stall occurred at
53 + 1/−2 MIAS. Again, right rudder and aileron
were required to prevent a significant left turning
tendency both clean and with gear and flaps extended. In the dirty configuration left and right
banks between 10◦ and 30◦ resulted in a stall velocity of 51 ± 1 MIAS independent of bank angle.
Wing drops of up to 45◦ were experienced. Both in
the full power and the power approach configurations the deck angle was estimated as 25 − 30◦ .
Summary The Bonanza, as originally equipped,
has excellent low speed handling and stall characteristics with good multiple stall warnings provided
by two stall/spin strips. As a result, the aircraft has
good short field characteristics.
At the test conditions, the VGs reduced the
power-off stall speed in the clean and dirty configurations by 13% and 6% respectively. In the dirty
power approach configuration a 7% decrease was
found. For all test cases good aileron and rudder
control were maintained. Previously, without the
VGs install, I was quite happy to come down short
final at 85-90 MIAS for a short field landing. With
the VGs, 75-80 MIAS is quite comfortable although
I generally use 82 MIAS because the stall warning
horn activates at 80 MIAS. These results make a
good short field aircraft even better.
As with anything, the reduction in stall speed
comes with a price. Specifically, VGs adversely affect flying qualities near stall, especially aerodynamic stall warning. Furthermore, the actual stall
is more aggressive as indicated by significant wing
drop. However, if you need to get in and out of
short fields or just want that extra margin above
stall, the cost may be acceptable.
Because of recent flight restrictions, characteristics at the optional 100 pound gross weight increase
were not tested. A separate article will discuss the
effect of vortex generators on cruise speed.
c
Copyright 2002
David F. Rogers. All rights reserved.